1. Field of the Invention
The present invention relates generally to the construction of three-dimensional soil nail walls comprising geosynthetic material facing systems, and optionally steel wire rope wales and soldier piles. A three-dimensional face wall including alternating, vertically-extending recessed and protruding portions is excavated that has a serpentine, ribbon-like arrangement. The geosynthetic materials are connected to the soil nails and then tensioned and drawn toward the slope so as to apply pressure to the soil face thereby creating zones of compressed soil within the slope.
2. Description of the Prior Art
Retaining wall systems are essential components of many excavation projects. Conventional soil nail (CSN) walls are commonly used in this capacity. CSN walls comprise a plurality of soil nails installed in a generally planar upright face of soil. Small diameter steel reinforcing bars and welded wire mesh are held up against the upright face, and a layer of shotcrete is applied over the top of the wire mesh and reinforcing bars.
CSN walls are typically constructed by first excavating a 3-5 feet high, self-supporting vertical slope along the wall alignment. Soil nails are then installed at points about two feet above the base of the excavation at 5-8 feet intervals. Soil nails are referred to as “passive inclusions” as tensile stress only develops within the steel threadbars of the soil nails after ground movement occurs. Next, drainage strips comprising rectangular plastic tubes surrounded by geotextile fabric are placed against the exposed soil face. These drainage strips generally extend from near the top of the slope all the way to the bottom and must be manipulated during each stage of the construction process.
Steel reinforcing bars are then attached to the threadbars protruding from the soil nails and serve as horizontal and vertical wales. Welded wire mesh is then attached to the soil nails and to the wales. During this process, the cement grout used to construct the soil nails cures and gains strength. Shotcrete is applied to the exposed slope covering the welded wire mesh and reinforcing bars. The shotcrete is allowed to cure over a period of 1-3 days. During this time, also known as the required stand up time, the slope must be self-supporting. The process is repeated until the entire slope is excavated and supported.
Shotcrete facing systems present numerous problems which make them unsuitable for installation under certain conditions. Steel-reinforced shotcrete soil nail wall facings are difficult to install in cold weather. The air used to propel the shotcrete onto the slope expands at the applicator nozzle and may cause the shotcrete to freeze if the ambient air temperature is below about 40° F. In addition, ground water can collect between the soil face and the shotcrete and lead to fracturing of the shotcrete around the soil nail heads.
Shotcrete also provides a very crude finish thereby making the CSN wall unaesthetic. Highly trained “shotcrete artisans” can be hired to create visually enhanced surfaces that resemble, for example, limestone blocks. However, the labor and materials required to provide such finishes are expensive. Thus, numerous methods have been devised to attach fascia walls on the outside of the shotcrete in order to reduce concerns over cracking of the shotcrete and to provide a more pleasing exterior finish.
Prior to assembly of the fascia wall, a second coating of reinforced shotcrete equipped with numerous anchors must be constructed to provide sufficient strength to support the fascia wall. This method of supporting the fascia wall is not desirable in that it is indirectly supported in the primary wall structure, increases the cost of wall construction, and provides another mechanism that could lead to wall failure.
CSN walls rely upon soil nails rather than foundations for vertical support. Thus, CSN wall facings are free to slump downward throughout their height as soil conditions change and may result in the wall top moving toward the excavation. The wall's appearance may remain marred even after repairs are performed.
Perhaps the greatest shortcoming of CSN walls is that they do not actively impose any pressure against the vertical slope. Face pressures only develop passively as ground movements occur. Reinforced shotcrete facings must bend to restrain soil from moving further, but in actuality, these facings present little bending strength and tend to readily crack. In many applications, only one layer of reinforcing steel is provided. As tension develops in the reinforcing steel, the shotcrete facing bulges outward resulting in the formation of cracks.
Soil arching is a phenomenon that increases the integrity of soil formations. It has been discovered that soil arching occurs within horizontal trenches when the trench sidewalls support a portion of the weight of the backfill through friction forces. The soil presses outward against the walls of the trench thereby inducing friction that partially supports the backfill and reduces loads placed on buried pipes. Purely analytical theories have been proposed to explain this condition based on elastic theory and basic soil properties. It has been discovered also that piles or piers separated by small gaps are capable of supporting vertical slopes. Exposed soil in the gaps between piers tends to fall away until a stable, concave surface develops behind the piers. This feature, indicative of soil arching, shows that zones of compressed soil develop behind the piers.
Another, more modern type of retaining wall is a wrapped-face mechanically stabilized earth (MSE) wall. Wrapped-face MSE walls employ strips of geosynthetic material which are horizontally laid. Backfill material is laid on top of the geosynthetic material strips, and the strips are wrapped around the face of the backfill forming a wall layer. Addition layers are constructed until the wall has attained the desired height. Tensile stresses develop passively in the geosynthetic material used in wrapped-face MSE retaining walls as ground movements occur.
Steep vertical slopes, such as those located on the west coast of the United States, can become unstable and collapse following periods of heavy rain. Seepage forces due to groundwater that flows through the soil and down to lower elevations can lead to a loss of slope strength. CSN walls are largely ineffective in these areas because they are not aesthetically pleasing. The passive support offered by CSN walls does not address the larger problem of preventing ground movement.
Thus, a real and unfulfilled need exists for an active retaining wall system that takes advantage of the soil arching phenomenon and does not possess the above-described problems which are inherent of conventional passive systems. There is also a need for an inexpensive wall which can be constructed in soil formations exhibiting shorter standup times than is acceptable with CSN wall construction.